1. Field of the Invention
This invention relates generally to hot-gas cleanup systems for feed gas to turbines; and more particularly, to a candle filter for such systems that includes an attached fail-safe/regenerator device, where the candle filter is a combination of a ceramic material within a supporting metal matrix.
2. Background Information
Modern industrial methods have resulted in a need for an apparatus that is capable of efficiently filtering high temperature combustion gases containing particulate material. In combustion turbine applications, for example, a combustion turbine uses energy generated from hot pressurized combustion gases produced by burning natural or propane gas, petroleum distillates or low ash fuel oil. When coal and other solid fuels are burned, particulates carried over from the combustion of such solid fuels can cause turbine blade erosion and fouling. An efficient system for filtering of such hot combustion gases would permit the use of such solid fuels. As another example, in conventional boiler operations, the boilers undergo routine shutdown for cleaning the fireside surfaces and for inspection. An efficient hot gas filtering system would greatly extend the life and operational time for a boiler to operate between inspections. Fouling of the fireside surface due to ash deposition and corrosion would be eliminated or minimized.
Also, as a key component in advanced coal-or biomass-based power applications, hot gas filtration systems protect the downstream gas turbine components from particle fouling and erosion, cleaning the process gas to meet emission requirements. When installed in either pressurized fluidized-bed (PFBC) combustion plants, pressurized circulating fluidized-bed combustion plants (PCFBC), or integrated gasification combined cycle plants (IGCC), lower downstream component costs are projected, in addition to improved energy efficiency, lower maintenance, and elimination of additional expensive fuel or flue gas treatment systems. As a critical component, long-term performance, durability, and life of the filter elements and associated high temperature gasket seals are essential to the successful operation of hot gas filtration systems in advanced combustion and gasification applications.
U.S. Pat. Nos. 5,185,019 and 5,433,771 (Haldipur et al. and Bachovchin et al., respectively) teach filtering apparatus including fail-safe/regenerator filter bodies and gasket assemblies that can be employed with conventional ceramic candle filters. In U.S. Pat. No. 5,876,471 (Lippert et al.), FIGS. 5 and 8 show state-of-the-art conventional ceramic monolithic candle filters and thin wall composite candle filters. The ceramic monolithic candle filters are usually thicker than the ceramic composite and/or ceramic filament wound, or metal and/or intermetallic filters. The ceramic composite filters have side walls about 1.0 mm to 5.0 mm thick and are made thinner by use of fibers and a rigidized matrix of SiC, alumina or aluminate silicate, and are generally lighter in weight than the monolithic ceramic filter elements. Fracture toughness was expected to be improved via use of the ceramic fiber reinforced composite matrix.
In all cases, the main candle filter is attached to a fail-safe/regenerator filter device which prevents particulate matter from traveling into the clean gas area of the pressure vessel if a ceramic filter element fails. Additionally, U.S. Ser. No. 09/263,436, filed on Mar. 4, 1999, now U.S. Pat. No. 6,123,746, Alvin et al., provided an improved rolled/layered gasket with a non-integral fail-safe/regenerator filter, and U.S. Ser. No. 09/393,561, filed on Sep. 10, 1999, now U.S. Pat. No. 6,273,925, Alvin et al., teaches an all-metal candle filter with reduced use of gaskets, and an integral fail-safe/regenerator filter device, similar to Bachovchin et al.
While these inventions provide advances in the art, enhanced strength, thermal fatigue resistant, oxidation and corrosion resistant candle filters are needed. Besides these specific type candle filters, others have provided porous ceramic honeycomb structures used as filters, as taught in U.S. patent application Ser. Nos. 4,364,760, 4,417,908 and 5,069,697 (Higuchi et al., Pitcher, and Hamaguchi et al., respectively). These structures have parallel channels through the body with alternating ends of the channels being sealed along both faces of the filter element, where gases generally pass through the channels parallel to the walls, except when passing through the channel walls to exit the filter. They are used to trap dust from gas on Diesel engines. In U.S. Pat. No. 4,448,833 (Yamaguchi et al.) a ceramic honeycomb structure, for use as a catalyst carrier or filter, was filled with a ceramic slurry containing organic solids, which after firing created fine interconnected holes within the supporting ceramic matrix, which vastly increased the filtering surface area. Strangman, in U.S. Pat. No. 4,867,639, utilized a soft ceramic, such as CaF2 or BaF2, to fill a honeycomb structure, as an abradable seal structure in gas turbines.
In U.S. Pat. No. 5,856,025 (White et al.) taught forming a ceramic-reinforced aluminum matrix composite by contacting a porous ceramic with molten aluminum-magnesium alloy, which infiltrated the ceramic mass to form an aluminum matrix. In U.S. Pat. No. 5,223,138 (Zievers et al.) a ceramic candle filter made of ceramic fibers and ceramic binder was bonded to an internal metallic perforate reinforcing member and the whole further coated with a ceramic material. Another supported candle filter is taught in U.S. Pat. No. 5,948,257 (Custer et al.) where ceramic fabric or yarn of, for example, mullite or silica is used as an inner and outer support layer for a reticulated ceramic foam interior.
Experience has shown that oxidation of non-oxide-based continuous fiber reinforced ceramic composites (xe2x80x9cCFCCxe2x80x9d), reticulated foam ceramic matrices, and clay bonded nonoxide-based ceramic matrices has resulted when these materials are used in the construction of porous candle filter elements which experienced long-term field operation, that is 500-3000 hours or greater of pressurized fluidized-bed combustion (xe2x80x9cPFBCxe2x80x9d) or pressurized circulating fluidized-bed combustion (xe2x80x9cPCFBCxe2x80x9d). Similarly, thick oxide-based ceramic monoliths were shown to be susceptible to thermal fatigue and/or shock during process operation. CFCC and filament wound matrices were shown to have low strength semi-densified flanges; have low strength and load bearing filter bodies; have the potential for embrittlement of the contained CFCC fibers; and to possibly debond along inserted plugs, seams, applied membranes, component layers, and the like. Additionally, in the development of porous metal filters, concern has been raised as to the long-term stability and corrosion resistance of the approximate 5 to 100 micrometer fibers or powder particles that are used to manufacture such all metal candle filter configurations.
There is still a need to develop higher reliability candle filter configurations for use in advanced coal-fired operation applications. Finally, it would be desirable that any improved filter be able to substitute into existing systems in the field.
Therefore, it is a main object of this invention to provide an improved candle filter having improved filtering capability and strength, and which is resistant to embrittlement, oxidation, and thermal and/or chemical degradation from sulfur, alkali, chlorides, and/or other contaminants found in process gas streams.
It is a further object of this invention to provide an improved candle filter that can easily substitute for existing units.
These and other objects are accomplished by providing an axially elongated candle filter element having an open end, a closed end and a porous wall defining an axially elongated inner volume, wherein the porous wall comprises an open metal structure disposed transverse to (90xc2x0) and surrounding the axis of the axially elongated inner volume, said open metal structure having an oxide coating on its surfaces, where the structure is filled with ceramic material to a density of from 30% to 70% of theoretical density, where at least one of the inner or outer surfaces of the filled structure has a membrane layer. Preferably, the candle filter element is tubular and the wall comprises an open metal honeycomb structure.
Fabrication of the metal-ceramic composite filter element with a hemispherical flange provides for ease of retrofit capabilities of the candles into existing filter operating systems with associated gasketing, fail-safe/regenerator, and mounting hardware. Should the standard hemispherical flange or a geometry thereof be fabricated as a dense metallic structural unit integral with the ceramic-filled metal structure, direct welding to either the fail-safe/regenerator or to the filter housing is feasible. Alternately, the ceramic-filled metal cylindrical tube manufactured without a flange can be welded directly to an integral metal fail-safe/regenerator and/or filter housing.
These metal-ceramic filter elements: preferably utilize a metallic or intermetallic honeycomb lattice to structurally reinforce the oxide-based ceramic filter matrix; mitigate catastrophic thermal fatigue failure of the oxide-based filter element through containment of the ceramic matrix within the honeycomb lattice; can be initially manufactured to near shape through the use of the metal honeycomb lattice, minimizing additional finishing after incorporation of the oxide-based material within the lattice, application of the outer and/or inner surface membrane, and final high firing; can be easily manufactured to the design tolerance specifications for immediate retrofit and use in existing field test facilities; and the increased thickness of the nonporous wall of the honeycomb lattice increases the extended life of the metal or intermetallic media over that of the fine grained particles or fibers currently used to manufacture porous metal media filter elements.